The term “pulp” generally refers to comminuted cellulosic material, such as wood chips, that have been processed in a digester vessel to separate the fibers in the wood chips. Chemicals, e.g., liquor, such as alkaline chemicals, are usually injected into the digester vessel to process and cook the cellulosic material to produce the pulp material. Residual chemicals tend to remain with the pulp material as the pulp material is discharged from the digester vessel.
After digestion, pulp material usually flows from the digester vessel to a pressurized diffuser vessel that washes the pulp material to remove the residual chemicals. A pressurized diffuser washer is typically a large vessel, e.g., 50 feet in height and generally houses a reciprocating screen assembly for washing pulp. Pulp with the residual chemicals can enter an annular chamber inside the pressure diffuser vessel and can fill the annular chamber extending much of the height of the pressure diffuser vessel. Wash water is typically injected into the annular chamber within the pressure diffuser vessel and flows through the pulp material to remove, e.g. displace, the residual chemicals from the pulp material. After the wash water flows through the pulp material the wash water is generally known as “wash filtrate” because it now contains the residual chemicals removed from the pulp material. The wash filtrate generally passes from the annular space of the pressure diffuser vessel to an internal screen assembly within the pressure diffuser vessel. The wash filtrate is typically discharged from a bottom outlet in the pressure diffuser vessel. The washed pulp material is typically discharged from the top of the pressure diffuser washer.
The screen assembly usually moves within the pressure diffuser vessel. Traditionally, the screen assembly moves reciprocally up and down during operation of the pressure diffuser washer. The movement of the screen assembly promotes the flow of pulp material through the annulus in the pressure diffuser vessel. Particularly, the upward movement allows the wash filtrate to enter the pulp material displacing dirty filtrate into the screen assembly and into the bottom of the vessel, where dirty filtrate may be removed. The downward movement of the screen assembly may assist in clearing the perforations or screen holes of fibers and particles that may be blocking the perforations.
The pulp material entering the pressure diffuser washer typically includes rocks and other debris. The debris generally moves with the pulp material as the pulp material moves up through the annular chamber in the pressure diffuser vessel. The debris can enter the water inlets on the outer wall of the annular chamber in the pressure diffuser vessel. The wash water inlets are typically gaps extending in a ring around the outer wall of the pressure diffuser vessel. If the gaps become clogged with rocks or other debris, the efficiency of the pulp wash process in the pressure diffuser vessel generally suffers. Removal of the rocks and debris from the wash water inlet gaps generally requires shutting down the pressure diffuser washer to stop wash water flow and allow the debris to be removed. Shutting down a pressure diffuser washer is expensive and interrupts the production of pulp. There is a long felt desire for a pressure diffuser washer that is less prone to loss of efficiencies and operating disruptions due to debris in the pulp material fed to the pressure diffuser vessel for washing.